P41. Flexion-extension radiographs underestimate instability in patients with single-level lumbar spondylolisthesis. Comparing flexion-supine imaging may be more appropriate

BACKGROUND CONTEXT Most spine surgeons use flexion-extension radiographs in their surgical decision-making for patients with lumbar spondylolisthesis. However, this method may underestimate the instability at a lumbar segment. Unfortunately, insurance companies often use radiology reports of flexion-extension radiographs to determine eligibility for lumbar fusion and often refuse appropriate care based on obsolete criteria. The utility of lateral dynamic films remains controversial in this population. To the authors' knowledge, no prior study has compared Slip Percentage (SP) in standing versus supine positioning. PURPOSE To compare flexion-extension (FE) with standing-supine (SS) and Flexion-Supine (FSu) views in providing an evaluation of segmental lumbar instability for single level spondylolisthesis. STUDY DESIGN/SETTING Retrospective cohort study. PATIENT SAMPLE Adult (>=18 years old) patients with single-level lumbar spondylolisthesis who underwent surgical treatment from 2016 to 2019. OUTCOME MEASURES Slip percentage differences and ventral instability. METHODS Consecutively collected adult (>=18 years old) patients with single-level lumbar spondylolisthesis who underwent surgical treatment from 2016 to 2019 were reviewed from a two-surgeon database. Anterior displacement (A) of a vertebra relative to the caudad vertebra was measured as a percentage of the sagittal length (B) of the caudad vertebral body (Slip Percentage, SP=100xA/B). Ventral instability was defined as an SP difference>8% as used in prior literature. Supine SP was calculated from sagittal Lumbar MRI. Chi-square/Fisher's Exact test and t-test were used for categorical and continuous variables. RESULTS Forty-four patients with lumbar spondylolisthesis who underwent surgery were identified. The cohort had a mean age of 57.3+-16.7 years and 66% of them were female. The majority had a slip grade of either 1 or 2 (Grade 1: 52.2%, Grade 2: 38.6%, Grade 3: 9.1%, Grade 4: 0%) and at the L4-5, 5-1 levels (L3-4: 2.3%, L4-5: 59.1%, L5-S1: 38.6%). The magnitude of change in SP was significantly highest in the FSu (12.7%) and SS (9.2%) groups, both of which were significantly higher compare with the traditional FE group (4.0%, P CONCLUSIONS Routine FE radiographs for lumbar spondylolisthesis may not be necessary and may underreport instability in single-level lumbar spondylolisthesis. SS and FSu parameters provide more sensitive measures of instability and may reduce unnecessary radiation exposure and costs. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.